It's hard to know where to start with this, because it's so basic and so general. What kind of look are you hoping to achieve? Have you ever taken photographs in a similar situation? I think I'd start by looking through the lighting basics tag here — that might give you some ideas.
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mattdmJan 25 '13 at 1:27

1

Use the type of lighting that provides more light. That is, any additional lighting. Open a curtain, use flash, turn on lights(but don't mix color temps). Also consider that you are the photographer. If the images would look better in a differently lit room, then by all means speak up!
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dpollittJan 25 '13 at 2:44

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Well: a room with no windows describes most photo studios. And you can certainly do it with two lights. Again, what kind of look are you going for?
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mattdmJan 25 '13 at 4:10

Well, you could technically take photos in a room nearly devoid of light with a high enough ISO setting. With ISO 12800, 25600, or even 51200 becoming more available and more useful, you can now take pictures in extremely dark settings.
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jrista♦Jan 25 '13 at 6:41

Fooey on your technicality. Consumer priced cameras have mediocre high ISO abilities. If you get an image, its got huge digital grain. Just turn on some lights.
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Pat FarrellJan 25 '13 at 15:06

...first, you didn't say what kind of mood you'd like. Also, you left the question quite general. Hence I'll try to give you some general tips:

flash (like the others said) bounced on the ceiling give a soft look, but usually this scares the babies which are so young, and I presume that the stark contrast of the unexpected powerful light compared with the darkness of the room could scary a lot the baby, hence, because of the possibility of a very unpleasant reaction from child, you will not have the occasion to shot many frames.

there is a logic in light. Why the client wants to shot the baby in there? In what pose? Is better to tell us these things. Perhaps she wants to photograph him asleep? Perhaps the room is important? Perhaps she doesn't want a simple head-and-shoulders portrait but an environmental portrait? Hence I think that's better to have a logic in light. While a ceiling-bounced flash might simulate the normal light which is in a room, perhaps is better to put one or more lampswithlampshades in order to give a different, more intimate mood and make the baby more cooperative. Also, I think that you can achieve a very good mood with this solution.

you can also combine the two solutions above but you must be careful at how the light will fall and also at different temperatures of light (a gel might help). Also in this case the flash, which will act like a fill flash, won't be so scary for the baby.

again, taking in consideration that you need to show also the room, you must have in mind the high ISOs (this applies especially if you use the lamps only), the fast aperture lens (eg. primes) (which gives you low DoF to separate your subject, keep the ISO and shutter at desired values) and in the case of using flash consider dragging the shutter in order to show also what's around.